1. Field of the Invention
The present invention relates to a multiple resolution image scanning module, particularly to an image scanning module of compact size, which allows for a scanning signal of nearly constant intensity under various resolutions.
2. Description of Related Art
Image scanning modules mainly comprise a light source, a reflective mirror, a lens, and a charge-coupled device (CCD). The number and size of the pixels of the CCD determine the optical properties of the image scanning module. For instance, flatbed scanners operate with charge-coupled devices which have 2700 pixels in one dimension. As the size of the scanned image is 8.5 inch in this dimension, a resolution of 300 dpi is attained. The resolution is a decisive factor for the quality of the image. For satisfactory quality, different kinds of images, like text, photographs, etc. require different resolutions (like 300 dpi, 400 dpi, 600 dpi, or 1000 dpi). So it is desirable to be able to choose the appropriate resolution on the image scanning module.
Conventional scanners are provided with image scanning modules for fixed resolutions. In a given conventional scanner, there is no way to change the scanned area or the resolution. For more than one resolution, several different image scanning modules have to be used.
In order to provide more flexibility, multiple resolution image scanning modules have entered the market, exemplified by Taiwan patents no. 232398 and 269416. Therein, image scanning modules are disclosed, having a reflective mirror and a constant distance between the scanned object and the charge-coupled device (CCD). Two lenses for two different resolutions are provided. The user switches between the lenses for the desired resolution. However, since the focal lengths of the two lenses are different, the lenses have to be mounted in different positions (as shown in FIG. 27). A large volume is needed, and thus the size of the image scanning module for multiple resolutions is increased. Furthermore, since the distance between the scanned object and the CCD is constant for different resolutions, at a high resolution the scanned area is smaller than at a low resolution, and the light intensity on the CCD becomes lower, leading to a lower signal, which is harder to process.